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ARTICLE
Molybdenum Nanoscrews: A Novel Non-coinage-Metal Substratefor Surface-Enhanced Raman Scattering
Di An1 . Yan Shen1 . Jinxiu Wen1 . Zebo Zheng1 . Jun Chen1 . Juncong She1 . Huanjun Chen1 . Shaozhi Deng1 .
Ningsheng Xu1
Received: 25 May 2016 / Accepted: 24 June 2016 / Published online: 31 August 2016
� The Author(s) 2016. This article is published with open access at Springerlink.com
Abstract For the first time, Mo nanoscrew was cultivated as a novel non-coinage-metal substrate for surface-enhanced
Raman scattering (SERS). It was found that the nanoscrew is composed of many small screw threads stacking along its
length direction with small separations. Under external light excitation, strong electromagnetic coupling was initiated
within the gaps, and many hot-spots formed on the surface of the nanoscrew, which was confirmed by high-resolution
scanning near-field optical microscope measurements and numerical simulations using finite element method. These hot-
spots are responsible for the observed SERS activity of the nanoscrews. Raman mapping characterizations further revealed
the excellent reproducibility of the SERS activity. Our findings may pave the way for design of low-cost and stable SERS
substrates.
Electronic supplementary material The online version of thisarticle (doi:10.1007/s40820-016-0104-6) contains supplementarymaterial, which is available to authorized users.
& Huanjun Chen
[email protected]
& Ningsheng Xu
[email protected]
1 State Key Laboratory of Optoelectronic Materials and
Technologies, Guangdong Province Key Laboratory of
Display Material and Technology, School of Electronics and
Information Technology, Sun Yat-sen University,
Guangzhou 510275, People’s Republic of China
123
Nano-Micro Lett. (2017) 9:2
DOI 10.1007/s40820-016-0104-6
Page 2
Graphical Abstract Mo nanoscrews are for the first time cultivated as a novel type of SERS substrate. The SERS activity
is originated from the electromagnetic field enhancements on the individual Mo nanoscrew, which is corroborated by
single-particle optical characterizations
Mo (200)
Laser
Mo (211)
Mo (110)
1500
1000
500
0
Inte
nsity
(a.u
.)20 30 40 50 60 70 80
2 Theta (degree)
10−4 M MB10−5 M MB10−6 M MB
2000 cps
×3
1200 1400
Inte
nsity
(a.u
.)
1600Raman shift (cm−1)
max
min
×6
hot spot
2 µm
Keywords Surface-enhanced Raman scattering (SERS) � Molybdenum nanoscrews � Quasi-one-dimensional
nanostructures � Electric field enhancements
1 Introduction
Raman spectroscopy is well known as a noninvasive, non-
labeling, and fingerprint-type sensing technique, which has
been widely utilized in environmental and life science
sensing and diagnostics [1–4]. However, traditional Raman
spectroscopies usually suffer from low sensitivity due to
the small Raman scattering cross sections of the analytes.
The discovery and development of SERS has brought
renascence to the Raman spectroscopy in terms of
remarkable enhanced Raman intensity from the analytes
[5–8]. The ultrasensitive sensing ability of the SERS is
primarily associated with the enhanced electromagnetic
field experienced by the analyte molecules in the vicinity
of nanostructures with light focusing property [9–11].
Coinage metal nanostructures are the most studied archi-
tectures as SERS substrates [6, 12–15]. These metallic
nanostructures can sustain plasmon resonances covering a
broad spectral range, giving rise to strong near field elec-
tromagnetic field enhancement in their nearby regions
under resonance excitation. Such field enhancement can
strengthen the Raman scattering of the analyte molecules
due to the fourth-power-dependence of the Raman intensity
on the incoming electric field.
On the other hand, in recent years people begin to show
great interests in non-coinage metal-based nanostructures
with SERS activity. Such nanostructures can benefit the
design and fabrication of high-performance sensors with
multiple functionalities that are derived from different
materials. For example, dielectric semiconductor nanos-
tructures with high refractive index, such as Si, ZnO, TiO2,
Fe2O3 [16–18], have been shown to exhibit intriguing
optical field enhancement properties and have attracted
many research interests as novel SERS substrates. In
comparison with their coinage metal counterparts, the
dielectric nanostructures usually exhibit excellent thermal
stability and biocompatibility.
Another type of materials that are considered to exhibit
SERS activity is transition metals [19–23]. Due to their
unique d-band electrons, the transition metals can effi-
ciently intermediate various chemical reactions under
proper electric bias, which makes them excellent catalysis
in various electrochemical reactions. It is therefore possible
to combine the SERS and electrocatalytic behaviors of the
transition metals as powerful manner to in situ monitor the
interfacial reactions. In these regards, various transition
metals, such as Ni, Fe, Ru, and Co, have been cultivated as
SERS substrates since the 1990s [19–24]. Some of them
have even been utilized for investigating the adsorption
behavior of organic molecules [23], opening up new ave-
nues for designing of novel surface characterization
techniques.
To the best of our knowledge, most of the previous
studies on transition metal-based SERS substrates focused
on thin films with nanostructured roughness while leaving
the elongated nanostructures less explored [25, 26]. In
comparison with other geometries, the elongated nanos-
tructures usually exhibit stronger electric field enhance-
ments. However, up to now there is still a lack of consensus
on the mechanisms governing the SERS activity of the
transition metal nanostructures, which demand for sys-
tematic investigations. On one hand, previous studies using
transition metal films with nanostructured surfaces sug-
gested that due to the unique electronic band structures of
the transition metals, charge transfer could happen between
the analyte molecules and metal surface. As a result, the
2 Page 2 of 10 Nano-Micro Lett. (2017) 9:2
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molecules can be strongly polarized and give rise to
enhanced Raman scattering. This is the so-called chemical
enhancement mechanism [19–24]. On the other hand, the
transition metal nanostructures should in principle localize
the electromagnetic field closely to their surfaces, which
can lead to strong electromagnetic field enhancement at
their vicinities. The Raman scattering of the adsorbed
molecules will therefore be enhanced. This is the electro-
magnetic enhancement mechanism, which is well studied
in noble metal nanostructures [12–15, 27].
Here, we demonstrate the observation of SERS activity
from quasi-one-dimensional crystalline molybdenum (Mo)
nanoscrew assemblies. The as-prepared Mo nanoscrews
consist of many screw threads on their surfaces and very
sharp heads at the ends. Such geometry is capable of con-
centrating the light field to enhance the Raman scattering of
the adsorbed methylene blue (MB) molecules. Raman map-
ping indicated that the nanoscrew substrate exhibited repro-
ducible SERS activity over an area of 100 9 100 lm2. The
underlying mechanisms of the SERS behavior of the Mo
nanoscrew assemblies were then systematically explored in
terms of single-particle dark-field scattering spectroscopy,
scanning near-field optical microscopy (SNOM), and
numerical electromagnetic simulations, which unveiled that
the amplified Raman signal were associated with the electric
field enhancement originated from the optical coupling
between the screw threads in each individual nanoscrew.
Besides, the electromagnetic coupling between different
nanoscrews can additionally contribute to the SERS activity
of the nanoscrew assemblies. The results obtained in our
study can not only demonstrate the SERS activity of the Mo
nanostructures, but also further our understanding on light-
matter interactions at the nanoscale.
2 Experimental Sections
2.1 Preparation of the Mo Nanoscrews
A Mo boat and a stainless steel substrate were separated
with 3 mm spacing and placed in the center of a vacuum
chamber. Gas mixture of argon and hydrogen was intro-
duced into the chamber when the vacuum in the chamber
was below 5 9 10-2 Torr. Thereafter, the Mo boat was
heated up to 1350 �C at a rate of 50 �C min-1. For
obtaining the Mo nanoscrews, the steel substrate was kept
at 1350 �C for another 15 min followed by a natural
cooling process [28].
2.2 Characterizations
Scanning electron microscope (SEM) images were con-
ducted using Zeiss Supra55 microscopes. Transmission
electron microscope (TEM) image was obtained by JEOL
JEM-2010 microscope operating at 200 kV. Electron dis-
persive spectrum (EDX) analysis was performed using
INCAX-max 250 analyzer integrated within the Zeiss
Supra55 microscope. Powder X-ray diffraction (XRD)
analysis was conducted using a Rigaku RINT2400 X-ray
diffractometer.
Dark-field scattering spectroscopy and imaging were
conducted using a home-made dark-field spectroscopy system
integrated with a white-light quartz tungsten halogen lamp as
excitation source [29]. The scattered spectra from the Mo
nanoscrews were corrected by first subtracting the back-
ground spectra taken from the adjacent regions without
nanoscrews and then dividing them with the calibrated
response curve of the entire optical system. Colored scattering
images were captured using the color digital camera (ART-
CAM-300MI-C, ACH Technology Co., Ltd., Shanghai)
mounted on the imaging plane of the microscope.
2.3 Sample Preparation for SERS Measurements
All of the four types of substrates, Mo nanoscrews, Mo thin
film-coated glass, quartz, and stainless steel, were immersed
in MB solution with specific molecule concentrations for
about 4 h to allow for the adsorption of the molecules. The
MB solution of various concentrations was prepared by dis-
solving different amounts of molecule powder into ethanol
(99.9 vol%). Afterwards the substrates were dried in air. To
prepare the sample for single-particle optical characteriza-
tions, the Mo nanoscrews were first scratched from the
stainless steel where they were grown and then dispersed into
the ethanol. The solution containing the nanoscrews was then
subjected to ultrasonication for 10 min to disperse the nano-
screws thoroughly. Thereafter, 10 lL of the suspension was
drop-casted onto the Mo- or ITO-coated glass with a pipet
and then dried naturally under ambient conditions. The dis-
tribution of the nanoscrews onto the substrate can be con-
trolled by tailoring the concentration of the nanoscrew
dispersion, which was diluted until well-dispersed nanoscrews
were found on the substrate. The substrates deposited with
Mo nanoscrews were then immersed in MB solution (10-4 M)
for about 4 h. The samples were dried in air at room
temperature before the Raman measurements. A pattern-
matching method was then utilized to locate the individual
nanoscrew for both morphology and optical characteriza-
tions [30].
2.4 Raman Measurements
The Raman spectra were acquired using a Renishaw inVia
Reflex confocal micro-Raman system equipped with a
Leica dark-field microscope. The excitation laser of
633 nm was focused onto the samples with a diameter of
Nano-Micro Lett. (2017) 9:2 Page 3 of 10 2
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*1 lm through a 509 objective (Leica, numerical aper-
ture: 0.8). The signal acquisition time for measuring the
Raman scattering from samples adsorbed with MB mole-
cules of 10-4, 10-5, and 10-6 M were 10, 30, and 60 s,
respectively. Raman maps were obtained by scanning the
samples under the microscope and recording the spectrum
from every single point. For obtaining the maps with large
area of 100 9 100 lm2, the scanning step was 1 lm. To
obtain the maps of single Mo nanoscrews, the scanning
step was 0.5 lm. The integrated intensity of the Raman
band between 1550 and 1700 cm-1 was used to create the
Raman maps.
2.5 AFM and SNOM Characterizations
AFM and SNOM characterizations were conducted using a
NeaSNOM scattering-type near-field optical microscope
(Neaspec GmbH). In a specific measurement, an AFM tip
coated with metal layer was illuminated using a visible
laser with wavelength of 633 nm. The tip was vibrated
vertically with frequency around 280 kHz. The backscat-
tered light from the tip was detected using a pseudo-
heterodyne interferometric manner, where the scattered
light was demodulated at third harmonic of the tip vibration
frequency. The optical and topography images of the
sample can be simultaneously obtained by scanning the
sample below the tip.
2.6 Numerical Calculations
The numerical calculations for the electromagnetic field dis-
tribution of an individual Mo nanoscrew were performed
using commercial software package (COMSOL Multiphysics
v4.3b). In a specific calculation, the screw threads were
modeled as casks. The nanoscrew was modeled by stacking
series of casks along the length direction of a free-standing
tapered cylinder. For simplicity, the supporting substrate was
not considered in the calculation. The length of the cylinder
was 1.3 lm; the thickness of the casks was 100 nm, and the
separation between adjacent casks was set as 2 nm. The
dielectric function of Mo was employed according to previ-
ous measurements [31]. In the simulation, a p-polarized light
with an incidence angle of 45� was launched into the box
containing the Mo nanoscrew to simulate the interaction
between the excitation light and the nanoscrew. The excita-
tion wavelength was 633 nm.
3 Results and Discussion
Figure 1a shows a typical SEM image of the as-prepared
sample, which indicates relatively uniform size and shape
distributions. The average density of the nanoscrews is
around 4–5 9 107 cm-2. The lengths of the nanoscrew
range from 2 to 6 lm, with maximum and minimum
diameters of over 500 and 50 nm at their bottoms and tips,
respectively. Each individual nanoscrew was consisted of
many small screw threads stacking along the longitudinal
direction with close separations. The EDS analysis of the
nanoscrews indicated that they are mainly composed of
metal Mo (Fig. 1b). One should note that small amount of
oxygen, with an intensity ratio of *1/40 to that of the Mo,
shows up in the EDS spectrum (Fig. 1b, inset). Such a
small amount of oxygen can be ascribed to contaminations
during the measurements. In our experiment, before the
EDS characterization the Mo nanoscrew was transferred
from the growth chamber into the SEM chamber for the
EDS measurement. During the transfer, the Mo nanoscrews
were exposed to atmospheric conditions, whereby the
oxygen and oxidized species (such as –OH) could be
adsorbed onto the metal surface. Due to these contamina-
tions, the oxygen peak will show up with weak intensity in
the EDS spectrum. Besides, in the EDS measurements due
to the limitation of the detector as well as collection
geometry, usually light elements with atomic number less
than 11 can be very difficult to measure reliably, if they can
be detected at all [32]. Therefore, the oxygen peak
observed from the EDS spectrum can only be used for
reference while not for quantitative analysis.
The morphologies and crystal structure of the nano-
screws can be further revealed by the TEM imaging and
XRD analysis (Fig. 1c, d). All of the diffraction peaks in
the XRD profile can be indexed as body-centered cubic
crystalline Mo (JCPDS 42-1120). The high-resolution
TEM image indicated that the small screw threads on the
nanoscrew were single-crystalline with a lattice spacing of
0.22 nm, which corresponded to the (110) plane of Mo. On
the basis of these characterizations, we conclude that the
nanoscrews are made up of pure metal Mo.
The unique structures of the Mo nanoscrews were
expected to be able to localize incidence light for
enhancing the Raman scattering of the adjacent molecules.
MB molecules were then utilized as probe analytes for
assessing the SERS performance of the Mo nanoscrews.
Besides the nanoscrew sample, three more types of sub-
strates, including rough Mo thin film, quartz, and stainless
steel, were used as references. The Mo thin film was pre-
pared by magneto sputtering, which could lead to surface
full of bumps and hollows. To evaluate the roughness, the
AFM was employed to measure the topography of the Mo
film. The root-mean-square average of height deviation on
the as-prepared Mo thin film was *3 nm (Fig. S1). Fig-
ure 2a gives the Raman spectra of the MB molecules
adsorbed onto the four substrates under 633-nm laser
excitation. Except the stainless steel, the other three sub-
strates gave clear Raman bands from the MB molecules,
2 Page 4 of 10 Nano-Micro Lett. (2017) 9:2
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which were assigned to m(CC)ring ? m(CNC)ring mode (1622,
1397 cm-1) and m(CC)ring mode (1434 cm-1), respectively
(Table 1) [33]. In comparison with the other two substrates,
the Raman intensity was evidently enhanced for Mo
nanoscrews adsorbed with MB molecules. These results
suggested that owing to their polycrystalline surface
(a) (c)
5 µm 2 nm
0.22 nm
(b)4000
3000
2000
1000
0
1500
1000
500
0
(d)Mo (200)
Mo (211)
Mo (110)
O120
80
40
00.4
0 5 10 15 20 20 30 40 50 60 70 80
0.6 0.8Energy (keV)
Energy (keV)
Inte
nsity
(a.u
.)
Inte
nsity
(a.u
.)
Inte
nsity
(a.u
.)
2Theta (degree)
2 µm
Fig. 1 Morphology and composition characterizations of the Mo nanoscrews. a Representative SEM image of the Mo nanoscrews. b EDS
spectrum of the Mo nanoscrews. Inset magnified EDS spectrum showing the oxygen peak. c TEM image of a typical Mo nanoscrew. Inset high-
magnification TEM image of area enclosed by the square. d XRD spectrum of the Mo nanoscrews
(c)
20 µm
Mo nanoscrewMo filmQuartzStainless steel
2000 cps
Inte
nsity
(a.u
.)
1200 1400Raman shift (cm−1)
1600
10−4 M MB10−5 M MB10−6 M MB
2000 cps
Inte
nsity
(a.u
.)
1200 1400
(b)(a)
Raman shift (cm−1)1600
1.0×103
4.8×104
×3
×6
Fig. 2 SERS activity characterizations of the Mo nanoscrews. a Raman spectra of MB molecules (10-4 M) adsorbed onto the Mo nanoscrews,
Mo thin film, quartz, and stainless steel. The excitation wavelength was 633 nm. b SERS spectra of the Mo nanoscrews adsorbed with 10-4 M
(green), 10-5 M (orange), and 10-6 M (light blue) of MB molecules. c Raman mapping image of the MB molecules (10-4 M) adsorbed onto the
Mo nanoscrews. The mapping corresponded to the integrated intensity of the Raman band between 1550 and 1700 cm-1. The mapping area was
over 100 9 100 lm2. (Color figure online)
Nano-Micro Lett. (2017) 9:2 Page 5 of 10 2
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features and porous architectures, the Mo nanoscrews
demonstrated superior SERS activity than that of the rough
Mo thin film.
We would like to discuss more on choose of the exci-
tation wavelength for the Raman characterizations. In the
Raman measurements, the Raman shifts of a specific type
of molecule does not change with different excitation
wavelengths. However, the intensities of the Raman bands
are strongly dependent on the excitation wavelengths. On
one hand, the intensity of the Raman scattering scales as
1/kex4 , where the kex is the excitation wavelength [34].
Hence excitation with a shorter wavelength can result in a
stronger Raman scattering. On the other hand, when the
excitation frequency is in resonance with an electronic
transition of the molecule under investigation, the Raman
scattering intensity of the molecule will be strongly
enhanced [35]. In such a manner, choosing an excitation
wavelength close to the absorption band of the molecule
can strengthen the Raman intensity. In our study, the
absorption band of the MB molecule centers around
650 nm (Fig. S2a). We therefore employed an excitation
wavelength of 633 nm for the Raman characterizations for
better demonstration. As a reference, we also conducted the
Raman measurements using 785-nm laser as the excitation
source, which is far from the MB absorption band. As
expected, the Raman intensity of the MB is much lower
than that under 633-nm excitation (Fig. S2b). However, in
comparison with other substrates, the excellent SERS
performance of the Mo nanoscrews still exists under the
785-nm excitation (Fig. S2c).
The SERS activity of the Mo nanoscrews was further
studied by immersing the substrates into MB molecule
solutions with various concentrations. Figure 2b shows that
the Raman intensity degrades as the molecule concentra-
tions are reduced. The main Raman bands of the MB
molecules can still be distinguished when the MB con-
centration reaches 10-6 M. Such performance is much
better than most reported values observed in transition
metal-based SERS substrates [19–26]. One of the crucial
parameters for characterizing the performance of a SERS
substrate is the reproducibility of the Raman signal. To
evaluate the SERS reproducibility of the Mo nanoscrews,
we conducted the Raman mapping within a typical sample
by monitoring the integrated intensity between 1550 and
1700 cm-1 over an area larger than 100 9 100 lm2
(Fig. 2c). The results clearly indicated that the Raman
spectra collected from different regions were comparable
with each other with a relative standard deviation value
of *0.3, suggesting the excellent SERS reproducibility of
the Mo nanoscrews.
To further reveal the origin of the SERS activity of the
nanoscrews, Raman measurements were conducted on
typical individual Mo nanoscrew. In a specific measure-
ment, the Mo nanoscrews were transferred to the Mo-
coated glass from the stainless steel substrate, which
allowed for both optical measurements and SEM imaging.
Another reason for choosing the Mo-coated glass as sup-
ported substrate was to provide direct comparison of the
SERS performance between the Mo nanoscrews and Mo
thin film. The SERS activity from the individual Mo
nanoscrews can be apparently seen by comparing the SEM
images and associated Raman maps (Fig. 3a–d). The dis-
tinct bright red regions with stronger Raman signal inten-
sity showed similar shapes as the profiles of the Mo
nanoscrews, suggesting that molecules adsorbed onto the
nanoscrews exhibited stronger Raman intensities than those
adsorbed onto the Mo thin film. These findings were con-
sistent with the above ensemble measurements. In order to
rule out the possibility that the Raman enhancement was
originated from the strong coupling between the Mo
nanoscrews and metallic Mo film with high refractive
index, Raman mapping characterizations were also con-
ducted for nanoscrews deposited onto indium tin oxide
(ITO)-coated glass substrates (Fig. 3e–h) under the same
excitation conditions. From the results, one can see that the
SERS activity persisted for individual Mo nanoscrew
deposited onto the ITO-coated glass substrate. These sin-
gle-particle Raman mapping characterizations clearly
indicated that the Raman enhancement behavior of the Mo
nanoscrews was contributed from the intrinsic SERS
activity of each nanoscrew in the assemblies.
For SERS substrate, the Raman enhancement factor
(EF) is a pivotal parameter for quantitatively characterizing
its SERS performance. However, in our study, it is difficult
to precisely determine the EF of the nanoscrews substrate
due to the random distribution of the nanoscrews in the
assemblies and the irregular shape of each individual
nanoscrew. To provide a preliminary estimation, we
choose to calculate the EF using the results from the above
single-particle measurements. Specifically, the EF of the
substrate can be calculated by referring to the 1622 cm-1
mode of the MB molecule according the following equa-
tion [36, 37],
Table 1 Band assignments of the Raman spectrum from the MB
molecules [33]
Raman shift (cm-1) Band assignment
1622 m(CC)ring ? m(CNC)ring1500 m(CC)ring1468 m(CC)ring1434 m(CC)ring1397 m(CC)ring ? m(CNC)ring1302 m(CC)ring
2 Page 6 of 10 Nano-Micro Lett. (2017) 9:2
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EF ¼ ISERS=NSERS
IRaman=NRaman
; ð1Þ
where IRaman and ISERS are the intensities of the 1622 cm-1
band for the normal (measured on the ITO-coated glass
adsorbed with the MB molecules) and SERS (measured on
an individual Mo nanoscrew adsorbed with the MB
molecules) spectra, NRaman is the number of molecules
probed for a normal Raman scattering, and NSERS is the
number of molecules probed on the Mo nanoscrew. In our
calculations, we employed the area intensity of the
1622 cm-1 band for both IRaman and ISERS. We approxi-
mated that the molecules were uniformly adsorbed onto the
ITO-coated glass and nanoscrew. Therefore, the ratio
between the NRaman and NSERS can be replaced by the ratio
between the areas of the ITO-coated glass (ARaman) and
nanoscrew (ASERS) within the laser spot.
The ARaman is the area of laser spot with a diameter of
1 lm. The ASERS can be estimated according to the struc-
ture of the nanoscrew. We modeled the nanoscrew as
stacking casks along its length direction (Fig. S3a, b). The
surface area of each cask can be obtained using the fol-
lowing equation (Fig. S3c),
Acask ¼Z
dA ¼Z
2pxds ¼2
Z Rþr
R
2prxdxffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffir2 � x� Rð Þ2
q
¼ 2 p2rRþ 2pr2� �
:
ð2Þ
The ASERS can then be expressed as
ASERS ¼ 0:5NAcask; ð3Þ
where N is the number of casks within the laser spot. The
factor 0.5 is due to the fact that only half of the cask is
exposed to the laser. According to the SEM characterizations
(Fig. 1a), we chose R and r as 200 and 50 nm, respectively.
There are about 10 casks were illuminated by the laser with a
spot size of 1 lm in diameter. The ratio of the Raman
intensity between the individual Mo nanoscrew and ITO-
coated glass was *1000 (Fig. 3f). Therefore, the EF of an
individual Mo nanoscrew can be calculated to be 700.
As mentioned above, the hot-spots with strong electro-
magnetic field enhancements are very important for the SERS
activity of a specific substrate. To unveil the hot-spots in the
Mo nanoscrews, we employed single-particle SNOM imaging
technique to measure the electromagnetic near field distri-
butions within an individual nanoscrew. The measurements
were based on apertureless scattering-type SNOM (s-SNOM),
whereby simultaneous morphology and near-field optical
images can be obtained via scanning the sample under the
atomic force microscope (AFM) tip [38, 39]. Typical topog-
raphy and associated near-field optical images of a Mo
nanoscrew were shown in Fig. 4a, b. Screw threads stacking
along the longitudinal direction of the nanoscrew can be seen
from the topography image, while the optical image was
manifested by bright spots of enhanced optical amplitudes
with respect to the background. These regions were corre-
sponded to the hot-spots and not ubiquitous on the nanoscrew.
Due to fact that both of the SEM (Fig. 4a, inset) and AFM
images had indicated small separations between the screw
threads on the nanoscrew, we attributed these hot-spots to the
near-field optical coupling between adjacent screw threads,
which was similar to those formed between noble metal
nanoparticles with small separations [27]. To prove this,
numerical electromagnetic simulation was performed to elu-
cidate the electric field distributions on the Mo nanoscrew. To
simplify the analysis without losing the physics, in the cal-
culations, the screw threads were modeled as casks. The
nanoscrew was modeled by stacking series of casks along the
length direction of a free-standing tapered cylinder (Fig. 4c).
The relatively large radius of the AFM tip precluded deter-
mination of the separation between adjacent screw threads.
Therefore, we arbitrarily chose a small value of 2 nm for the
separations between adjacent casks in a specific calculation.
9×103
1.3×103
9.8×103
1.4×103
22.1×103
2.5×101
5.6×103
3.1×101
(a) (b) (c) (d)
(e) (f) (g) (h)
2 µm 2 µm 2 µm 2 µm
2 µm 2 µm4 µm 2 µm
Fig. 3 Raman mapping of the individual Mo nanoscrews modified with MB molecules (10-4 M). a–d SEM images and corresponding Raman
maps of the Mo nanoscrews deposited onto the Mo film-coated glass substrate. e–h SEM images and corresponding Raman maps of the Mo
nanoscrews deposited onto the ITO-coated glass substrate. The excitation wavelength was 633 nm. The mapping corresponded to the integrated
intensity of the Raman band between 1550 and 1700 cm-1
Nano-Micro Lett. (2017) 9:2 Page 7 of 10 2
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The calculated near field distributions evidently showed that
the gap regions between two neighboring casks could confine
the electric field to form hot-spots (Fig. 4c). Furthermore, the
sharp tip of the nanoscrew was also shown to strongly con-
centrate the incidence light. These results agreed with the
previous experimental measurements.
The strong hot-spots can be further manifested by the far-
field scattering properties of the nanoscrews. Due to the
localized enhanced electric field, the nanoscrew can be
strongly polarized. The oscillating polarized charges in the
nanoscrew can therefore induce strong light scattering into the
far-field, giving rise to enhanced scattering bands in the visible
and near-infrared regions (Fig. 4d). Such strong light scatter-
ing behavior of the nanoscrew can be also seen from its
scattering image, which shows vivid color appearance
(Fig. 4d, inset). These responses of the nanoscrew also suggest
its potential in future visible and near-infrared photonic and
optoelectronic applications.
The optical near-field results clearly reveal the hot-spots
in an individual Mo nanoscrew, which are responsible for
the SERS activity of the nanoscrew assemblies. On the
other hand, in the assembly the separations between
different nanoscrews are small (Fig. 1a), whereby effective
electromagnetic coupling between adjacent Mo nanoscrews
can be induced. Such coupling effects can give rise to even
stronger electric field enhancements, which will addition-
ally contribute to the Raman enhancements of the nano-
screw assemblies. In our current study, we intend to
demonstrate the Mo nanoscrews as a novel type of SERS
substrate and elucidate the underlying mechanisms. In this
regard, it is more important for us to characterize the
intrinsic SERS activity from an individual Mo nanoscrew
and evaluate its ability to localize the electromagnetic field.
Therefore, we did not pay much attention to optimize the
SERS performance by taking advantage of the coupling
between different nanoscrews in the assemblies.
4 Conclusion
In conclusion, we have cultivated the Mo nanoscrews as
non-coinage-metal SERS substrate. By using Raman
mapping technique, we have elucidated the SERS activity
of the nanoscrews with good reproducibility. The SERS
(d)1.0
0.8
0.6
0.4
0.2
0
400 600 800Wavelength (nm)
Inte
nsity
(a.u
.)
max
minmax
min
max
min
(a) (b)
(c)
2 µm
2 µm
200 nm
2 µm
Fig. 4 Characterizations of the hot-spots on an individual Mo nanoscrew. a AFM topography image of a typical Mo nanoscrew. Inset
corresponding SEM image of the nanoscrew. b Optical near-field amplitude (3rd harmonics) at excitation of 633 nm recorded in the same area as
that in a. c Calculated electric-field-magnitude enhancement contour of an individual Mo nanoscrew. The nanoscrew was excited by a p-
polarized light at an incidence angle of 45�. The polarization of the incidence light had a component parallel to the length direction of the
nanoscrew. The excitation wavelength was 633 nm. d Dark-field scattering spectrum of the Mo nanoscrew. Inset dark-field scattering image of
the Mo nanoscrew
2 Page 8 of 10 Nano-Micro Lett. (2017) 9:2
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Page 9
effect is attributed to the intrinsic SERS activity of each
nanoscrew in the assembly. Through high-resolution
SNOM and SEM measurements, we have shown that the
Raman enhancement behavior of the individual nanoscrew
is associated with its unique structure. The as-prepared Mo
nanoscrews were composed of many small screw threads
stacking along the longitudinal direction with small sepa-
rations. Such small gaps can act as hot-spots to localize the
incidence light for strong electric field enhancements,
giving rise to the observed SERS activity. We strongly
believe that the results obtained in this study can on one
hand help extend the SERS substrates to other low-cost and
stable material systems, and on the other hand fertilize the
functionalities of transition metal-based nanostructures.
Acknowledgments This work was supported by the National Natural
Science Foundation of China (Grant Nos. 11474364, 51202300,
51290271), the National Key Basic Research Program of China
(Grant Nos. 2013CB933601, 2013YQ12034506), the Guangdong
Natural Science Funds for Distinguished Young Scholar (Grant No.
2014A030306017), the Guangdong Special Support Program, the
Doctoral Fund of Ministry of Education of China (Grant No.
20120171120012), the Program for Changjiang Scholars and Inno-
vative Research Team in University (Grant No. IRT13042), and the
Fundamental Research Funds for the Central Universities.
Open Access This article is distributed under the terms of the
Creative Commons Attribution 4.0 International License (http://crea
tivecommons.org/licenses/by/4.0/), which permits unrestricted use,
distribution, and reproduction in any medium, provided you give
appropriate credit to the original author(s) and the source, provide a
link to the Creative Commons license, and indicate if changes were
made.
References
1. U. Gala, H. Chauhan, Principles and applications of Raman
spectroscopy in pharmaceutical drug discovery and development.
Expert Opin. Drug Discov. 10(2), 187–206 (2015). doi:10.1517/
17460441.2015.981522
2. C. Krafft, J. Popp, The many facets of Raman spectroscopy for
biomedical analysis. Anal. Bioanal. Chem. 407(3), 699–717
(2015). doi:10.1007/s00216-014-8311-9
3. J. Surmacki, J. Musial, R. Kordek, H. Abramczyk, Raman
imaging at biological interfaces: applications in breast cancer
diagnosis. Mol. Cancer 12, 48 (2013). doi:10.1186/1476-4598-
12-48
4. H. Edwards, T. Munshi, I. Scowen, A. Surtees, G.T. Swindles,
Development of oxidative sample preparation for the analysis of
forensic soil samples with near-IR Raman spectroscopy. J. Ra-
man Spectrosc. 43(2), 323–325 (2012). doi:10.1002/jrs.3031
5. J. Kneipp, H. Kneipp, K. Kneipp, SERS—a single-molecule and
nanoscale tool for bioanalytics. Chem. Soc. Rev. 37(5),1052–1060 (2008). doi:10.1039/b708459p
6. W. Xie, S. Schlucker, Rationally designed multifunctional plas-
monic nanostructures for surface-enhanced Raman spectroscopy:
a review. Rep. Prog. Phys. 77(11), 116502 (2014). doi:10.1088/
00344885/77/11/116502
7. G. Sarau, B. Lahiri, P. Banzer, P. Gupta, A. Bhattacharya, F.
Vollmer, S. Christiansen, Enhanced Raman scattering of gra-
phene using arrays of split ring resonators. Adv. Opt. Mater. 1(2),151–157 (2013). doi:10.1002/adom.201200053
8. Y.S. Huh, A.J. Chung, D. Erickson, Surface enhanced Raman
spectroscopy and its application to molecular and cellular anal-
ysis. Microfluid. Nanofluid. 6(3), 285–297 (2009). doi:10.1007/
s10404-008-0392-3
9. Y. Fang, N.-H. Seong, D.D. Dlott, Measurement of the distri-
bution of site enhancements in surface-enhanced Raman scat-
tering. Science 321(5887), 388–392 (2008). doi:10.1126/science.
1159499
10. H. Xu, E.J. Bjerneld, M. Kall, L. Borjesson, Spectroscopy of
single hemoglobin molecules by surface enhanced Raman scat-
tering. Phys. Rev. Lett. 83(21), 4357–4360 (1999). doi:10.1103/
PhysRevLett.83.4357
11. D.-K. Lim, K.-S. Jeon, H.M. Kim, J.-M. Nam, Y.D. Suh,
Nanogap-engineerable Raman-active nanodumbbells for single-
molecule detection. Nat. Mater. 9(1), 60–67 (2009). doi:10.1038/
nmat2596
12. M.J. Banholzer, J.E. Millstone, L.D. Qin, C.A. Mirkin, Rationally
designed nanostructures for surface-enhanced Raman spec-
troscopy. Chem. Soc. Rev. 37(5), 885–897 (2008). doi:10.1039/
b710915f
13. D.-K. Lim, K.-S. Jeon, J.-H. Hwang, H. Kim, S. Kwon, Y.D. Suh,
J.-M. Nam, Highly uniform and reproducible surface-enhanced
Raman scattering from DNA-tailorable nanoparticles with 1-nm
interior gap. Nat. Nanotechnol. 6(7), 452–460 (2011). doi:10.
1038/nnano.2011.79
14. J.X. Wen, H.B. Zhang, H.J. Chen, W.H. Zhang, J. Chen,
Stretchable plasmonic substrate with tunable resonances for
surface-enhanced Raman spectroscopy. J. Opt. 17(11), 114015(2015). doi:10.1088/2040-8978/17/11/114015
15. S. Chen, P. Xu, Y. Li, J. Xue, S. Han, W. Ou, Y. Ding, W. Ni,
Rapid seedless synthesis of gold nanoplates with micro-
scaled edge length in a high yield and their application in SERS.
Nano-Micro Lett. 8(4), 336–346 (2016). doi:10.1007/s40820-
016-0092-6
16. S.M. Wells, I.A. Merkulov, I.I. Kravchenko, N.V. Lavrik, M.J.
Sepaniak, Silicon nanopillars for field-enhanced surface spec-
troscopy. ACS Nano 6(4), 2948–2959 (2012). doi:10.1021/
nn204110z
17. Q. Liu, L. Jiang, L. Guo, Precursor-directed self-assembly of
porous ZnO nanosheets as high-performance surface-enhanced
Raman scattering substrate. Small 10(1), 48–51 (2014). doi:10.
1002/smll.201300440
18. L. Li, T. Hutter, A.S. Finnemore, F.M. Huang, J.J. Baumberg,
S.R. Elliott, U. Steiner, S. Mahajan, Metal oxide nanoparticle
mediated enhanced Raman scattering and its use in direct mon-
itoring of interfacial chemical reactions. Nano Lett. 12(8),4242–4246 (2012). doi:10.1021/nl302029p
19. Z.Q. Tian, B. Ren, D.-Y. Wu, Surface-enhanced Raman scatter-
ing: from noble to transition metals and from rough surfaces to
ordered nanostructures. J. Phys. Chem. B 106(37), 9463–9483(2002). doi:10.1021/jp0257449
20. B. Ren, Q.J. Huang, W.B. Cai, B.W. Mao, F.M. Liu, Z.Q. Tian,
Surface Raman spectra of pyridine and hydrogen on bare
platinum and nickel electrodes. J. Electroanal. Chem.
415(1–2), 175–178 (1996). doi:10.1016/S0022-0728(96)
01004-2
21. P.G. Cao, J.L. Yao, B. Ren, B.W. Mao, R.A. Gu, Z.Q. Tian,
Surface-enhanced Raman scattering from bare Fe electrode sur-
faces. Chem. Phys. Lett. 316(1–2), 1–5 (2000). doi:10.1016/
S0009-2614(99)01207-5
22. D.Y. Wu, Y. Xie, B. Ren, J.W. Yan, B.W. Mao, Z.Q. Tian,
Surface enhanced Raman scattering from bare cobalt electrode
Nano-Micro Lett. (2017) 9:2 Page 9 of 10 2
123
Page 10
surfaces. Phys. Chem. Comm. 4(18), 89–91 (2001). doi:10.1039/
b105667k
23. P.G. Cao, R.A. Gu, B. Ren, Z.Q. Tian, Surface-enhanced Raman
scattering of pyridine on platinum and nickel electrodes in non-
aqueous solutions. Chem. Phys. Lett. 366(3–4), 440–446 (2002).
doi:10.1016/S0009-2614(02)01663-9
24. J.S. Gao, Z.Q. Tian, Surface Raman spectroscopic studies of
ruthenium, rhodium and palladium electrodes deposited on glassy
carbon substrates. Spectrochim. Acta A 53(10), 1595–1600
(1997). doi:10.1016/S1386-1425(96)01855-0
25. J.-L. Yao, J. Tang, D.-Y. Wu, D.-M. Sun, K.-H. Xue, B. Ren, B.-
W. Mao, Z.-Q. Tian, Surface enhanced Raman scattering from
transition metal nano-wire array and the theoretical consideration.
Surf. Sci. 514(1–3), 108–116 (2002). doi:10.1016/S0039-
6028(02)01615-1
26. G. Sauer, G. Brehm, S. Schneider, H. Graener, G. Seifert et al.,
Surface-enhanced Raman spectroscopy employing monodisperse
nickel nanowire arrays. Appl. Phys. Lett. 88(2), 023106 (2006).
doi:10.1063/1.2162682
27. J. Wu, Y.J. Xu, P.Y. Xu, Z.H. Pan, S. Chen, Q.S. Shen, L. Zhan,
Y.G. Zhang, W.H. Ni, Surface-enhanced Raman scattering from
AgNP-graphene-AgNP sandwiched nanostructures. Nanoscale
7(41), 17529–17537 (2015). doi:10.1039/C5NR04500B
28. Y. Shen, N.S. Xu, S.Z. Deng, Y. Zhang, F. Liu, J. Chen, A Mo
nanoscrew formed by crystalline Mo grains with high conduc-
tivity and excellent field emission properties. Nanoscale 6(9),4659–4668 (2014). doi:10.1039/c3nr06811k
29. H. Wang, P. Liu, Y.L. Ke, Y.K. Su, L. Zhang, N.S. Xu, S.Z.
Deng, H.J. Chen, Janus magneto-electric nanosphere dimers
exhibiting unidirectional visible light scattering and strong elec-
tromagnetic field enhancement. ACS Nano 9(1), 436–448 (2015).
doi:10.1021/nn505606x
30. H.J. Chen, Z.H. Sun, W.H. Ni, K.C. Woo, H.-Q. Lin, L.D. Sun,
C.H. Yan, J.F. Wang, Plasmon coupling in clusters composed of
two-dimensionally ordered gold nanocubes. Small 5(18),2111–2119 (2009). doi:10.1002/smll.200900256
31. E.D. Palik, Handbook of Optical Constants of Solids (Academic
Press, Boston, 1985)
32. J. Konopka, Options for Quantitative Analysis of Light Elements
by SEM/EDS, Technical Note 52523 (Thermo Fisher Scientific,
Madison, 2013)
33. S.H.D.A. Nicolai, P.R.P. Rodrigues, S.M.L. Agostinho, J.C. Rubim,
Electrochemical and spectroelectrochemical (SERS) studies of the
reduction ofmethylene blue on a silver electrode. J. Electroanal. Chem.
527(1–2), 103–111 (2002). doi:10.1016/S0022-0728(02)00832-X34. G. Herzberg, Molecular Spectra and Molecular Structure II
Infrars and Raman Spectra of Polyayomic Molecules (Van
Nostrand Reinhold, New York, 1945)
35. R.S. Drago, Physical Methods in Chemistry (Saunders,
Philadelphia, 1977)
36. W. Li, P.H.C. Camargo, X. Lu, Y.N. Xia, Dimers of silver
nanospheres: facile synthesis and their use as hot spots for sur-
face-enhanced Raman scattering. Nano Lett. 9(1), 485–490
(2009). doi:10.1021/nl803621x
37. P.H.C. Camargo, L. Au, M. Rycenga, W. Li, Y.N. Xia, Mea-
suring the SERS enhancement factors of dimers with different
structures constructed from silver nanocubes. Chem. Phys. Lett.
484(4–6), 304–308 (2010). doi:10.1016/j.cplett.2009.12.002
38. F. Keilmann, R. Hillenbrand, Near-field microscopy by elastic
light scattering from a tip. Philos. Trans. R. Soc. Lond. Ser. A
362(1817), 787–805 (2004). doi:10.1098/rsta.2003.1347
39. N. Ocelic, A. Huber, R. Hillenbrand, Pseudoheterodyne detection
for background-free near-field spectroscopy. Appl. Phys. Lett.
89(10), 101124 (2006). doi:10.1063/1.2348781
2 Page 10 of 10 Nano-Micro Lett. (2017) 9:2
123